The control of superheater temperature may be accomplished through a spray attemperator and a control system that positions the spray flow control valve. Such a control system is based on steam temperature error. It has been found, however, that such a control system has a poor response time and can result in potentially unstable control.
The systems usually employed for boilers connected to steam turbines operating at a constant throttle pressure utilize various control approaches to achieve more stable operation and faster response time. For example, a system might control the attemperator outlet temperature to a set point which is adjusted based on outlet steam temperature error. Alternatively, a system could include a feed forward program for spray flow with the spray flow controlled to a demand determined by the program along with steam temperature error. Either of these approaches is quite satisfactory for applications involving constant pressure operation but is inadequate for variable pressure applications due to the variations in temperature within the superheater and the large changes in spray flow caused by variations in operating pressure. These inadequacies are further increased in those applications which include a pressure control valve within the superheater since such a control valve increases the number of possible operating conditions.
Because of the foregoing, it has become desirable to develop a superheater spray control system that can be utilized in variable pressure applications and can compensate for temperature variations within the superheater.